Microwave backhaul has been extensively used to connect base stations to the corresponding base station controllers for several years (for example, Non Patent Document 1).
However, currently, wireless networks are evolving from supporting voice-only to supporting both voice and high-speed data services. Thus, there will be an increasing need for bandwidth capacity at base stations and microwave backhaul.
Line-of-sight (LoS) MIMO can be considered as a candidate solution for high capacity microwave backhaul (for example, Non Patent Document 2). In these systems, due to the lack of multipath scattering, the antenna separation depends on communication distance to achieve space multiplexing.
For example, in a LoS MIMO system as illustrated in FIG. 1, 2*2 (two-by-two) form communication is performed by using transmitting antennas Tx1 and Tx2 and receiving antennas Rx1 and Rx2. Distance d between the transmitting antennas Tx1 and Tx2 which is necessary for separating the transmitting antennas Tx1 and Tx2 depends on communication distance L, as represented by formula (1).d=√{square root over (λL/2)}  (1)
Here, λ represents wavelength. In a case where the communication distance L is 400 m and communication frequency is 2.4 GHz, the distance d becomes about 5 m. In a case where the communication distance L is 400 m and communication frequency is 60 GHz, the distance d becomes about 1 m. Therefore, the antenna separation (the distance d) is relatively large and usually not available for compact devices. Furthermore, if the antenna separation shrinks, the channel capacity degrades drastically.
Recently, the study of orbital angular momentum (OAM) is very hot in high capacity optical communication (for example, Non Patent Document 3). OAM, similar to polarization (Spin Angular Momentum (SAM)), is also a fundamental property of electromagnetic waves.
As illustrated in FIG. 2, an electromagnetic wave having OAM has a spiral wavefront, and represents a linear phase delay with azimuthal angle OAM mode 1 (1=±1, ±2, . . . ) represents that there is a phase delay of 21π during one cycle (physical one cycle). The phase delay is represented in an electric angle.
In the configuration of optical communication as illustrated in FIG. 3, OAM signals are superposed with respect to a single optical axis by using optical combiners (for example, Non Patent Document 3).
Since electromagnetic waves having different OAM modes are orthogonal to each other, high capacity is achieved due to multiple orthogonal channels.
However, it is difficult for a Radio Frequency (RF) signal to superpose OAM channels with respect to a single optical axis. Thus, it is difficult to multiplex the RF signal.
Non-Patent Document 1: Mobile Backhaul: Fiber vs. Microwave, Case Study Analyzing Various Backhaul Technology Strategies, Tzvika Naveh. [Searched on Jan. 29, 2013] Internet (http://www.ceragon.com/files/ceragon_mobile_backhau_fiber_microwave_white_paper.pdf)
Non-Patent Document 2: C. Sheldon, E. Torkildson, M. Seo, C. P. Yue, U. Madhow, and M. Rodwell, “A 60 GHz line-of-sight 2×2 MIMO link operating at 1.2 Gbps,” in Proc. IEEE Antennas Propag. Soc. Int. Symp. (AP-S 2008), July 2008.
Non-Patent Document 3: J. Wang, J.-Y. Yang, I. M. Fazal, N. Ahmed, Y. Yan, H. Huang, Y. Ren, Y. Yue, S. Dolinar, M. Tur, and A. E. Willner, “Terabit free-space data transmission employing orbital angular momentum multiplexing,” Nature Photonics, vol. 6, pp. 488-496, July 2012.